1. Field of the Invention
The present invention relates to an optical glass, and in particular to an optical glass consisting of improved components to provides high stability against devitrification and high mass production property. The optical glass is also free from environmental and human harmful components, and is suitable for molding at a low temperature.
2. Description of Prior Art
Currently, the performance requirements for photoelectric products, such as digital cameras, are continually increased; the dimensions of these products are required to be further reduced; and the image quality requirements are also continually increased. To meet these requirements, the lens design has become increasingly complicated Aspheric lenses, together with low-cost spherical lenses, must be employed in an optical system to meet the low cost and high quality requirements. Among the known methods for manufacturing aspheric lenses, grinding and polishing are not only very time inefficient, leading to low productivity, but also increases cost. Therefore, precision press molding technology has now become a major and necessary technology for aspheric lens manufacture.
In the case of known mold materials for precision molding, there arises a problem that the higher the molding temperature is, the more oxidation or deterioration the material of the mold itself may have, thus resulting in difficulty in maintenance of the surface accuracy of the mold and in mass production of lenses by the precision molding. Accordingly, to effectively increase the service life of a mold, the precision molding temperature should be reduced.
On the other hand, the precision molding is generally carried out at a high temperature ranging from about 30 to 60° C., which is above the yield point temperature (At) of glass. When the molding temperature exceeds 620° C., OH molecules that adhere to the surface of the glass will react with the mold and ends up decomposing. This decomposition reaction leaves numerous bubbles on the surface of molded glass lenses. Thus, not only does it become difficult to maintain the degree of precision of the transfer surface of the optical part that is molded, but damage is done to the surface precision of the mold material as well, compromising mass production. Therefore, when making an aspheric glass lens with precision press molding, the yield point temperature of the glass used should be as lower as possible and at least lower than 590° C., so as to ensure mass production. Glasses exhibiting a yield point temperature higher than 590° C. can be used to manufacture spherical lenses, prisms and other optical elements. Spherical lenses can be finally formed by grinding and polishing glass performs that are manufactured by reheating and press molding.
In recent years, it has been acknowledged that some glass components, such as PbO and As2O3, are harmful to human bodies and the environment. Accordingly, optical device manufacturers tend to avoid using these poisonous components in optical glasses. Japanese Patent Unexamined Publication Nos. 06-056463 and 04-002628 each disclose a conventional optical glass exhibiting a yield point temperature lower than 590° C. and a refractive index in the range of 1.66-1.72. The components of these conventional optical glasses contain a large amount of lead oxide, PbO, which is harmful to human bodies and the environment. Further, precision press molding is ordinarily conducted at a high temperature and in a nonreactive atmosphere or a weakly reducing atmosphere to prevent oxidation of the mold. When the glass components containing a large amount of lead oxide are precision pressed, lead oxide is reduced on the glass surface and precipitates onto the glass surface as metallic lead. Further, with repeated press molding, the precipitating metallic lead adheres to the molding surface of the mold, decreasing the precision of the molding surface and eventually causing loss of the surface precision of the transfer surface of the molded product. Thus, maintenance is required to remove the metallic lead adhering to the mold, compromising mass production. Japanese Patent Unexamined Publication No. 10-316448 discloses an optical glass containing arsenic dioxide, As2O3, which is also poisonous to human bodies. However, simply replacing lead oxide or arsenic dioxide with one or more other components theoretically cannot completely obtain corresponding optical and technical characteristics of lead oxide or arsenic dioxide. Therefore, it is necessary to dramatically reformulate the glass components and their weight percentages.
To prevent the glass from devitrifying during the manufacturing process, which results in poor glass appearance, the liquidus temperature (LT) of glass is preferably as low as possible. That is, the greater the difference between the operating temperature and the glass liquidus temperature is, the smaller the likelihood that the glass tends to devitrify during manufacturing will be. Glass having a high liquidus temperature is liable to devitrify and is therefore poor in mass production. Japanese Patent Unexamined Publication No. 08-259257 discloses a glass having a liquidus temperature greater than 1000° C., and is thus poor in stability.
In addition, it is preferred to have the melting temperature of glass raw materials as low as possible, in order to reduce energy consumption and prolong the service life of furnace used to glass related operations. Japanese Patent Laid-open No. 2000-247676 discloses a glass that has raw materials molten at a high temperature of 1500° C., which consumes a large amount of energy and thus increases the cost.